Several investigators have found that tobacco quality is improved by reducing its protein content. Although it is relatively easy to remove protein from uncured tobacco leaf, there are disadvantages to removing protein before curing. The major problem is that protein broken down during curing can form flavor compounds that are important contributors to the organoleptic properties of the smoke. Another disadvantage is that efficient extraction of green leaf usually necessitates tobacco structural changes which make it difficult to produce shredded tobacco suitable for use as a cigarette filler.
Partial removal of protein from cured tobacco can be accomplished by extraction with water, with the efficiency of the extraction improving as the particle size is reduced. However, for shredded tobacco of the size normally used for cigarette manufacture, most of the protein cannot be extracted by water alone. Several inventors have found that proteolytic enzymes will break down tobacco protein into readily soluble fragments and that strip or cut tobacco can be treated by such enzymes. Thus Gaisch et al. (U.S. Pat. No. 4,407,307) described the removal of protein from tobacco strips in an aqueous solution of a proteolytic enzyme whereby insoluble proteins are decomposed into soluble fragments. The extract is separated from the tobacco and inoculated with a yeast culture, which, as it grows, removes the soluble protein fragments in the extract by metabolic assimilation. After removal of the yeast, the protein-free extract is concentrated and added back to the tobacco strips. Bernasek et al. (U.S. Pat. No. 4,887,618) describe a process in which tobacco is first extracted with water. The tobacco residue remaining after extraction is separated from the solution, mixed with water and treated with a proteolytic enzyme. The protein-reduced tobacco is separated from the enzyme solution, rinsed and dried. The water extract is concentrated and added back to the protein reduced tobacco. The advantage described by Bernasek et al. for this process is that the water soluble flavor components of tobacco and the nicotine can be retained in the final product.
The above described processes rely on protease enzymes alone to remove protein from tobacco material. Our own investigations have found that enzymes which efficiently remove protein from tobacco are expensive, while those enzymes which are available in commercial quantities at a reasonable price, are much less efficient for protein removal. Poulose et al. (U.S. Pat. No. 4,716,911) has also realized this disadvantage and proposed using either an alkali or a combination of a protease and a non-protease depolymerase to effect protein removal in an overall processing scheme similar to that of Gaisch et al. However, we have found that alkaline solutions at the strengths quoted by Poulose et al. may have a deleterious effect on the physical structure of the tobacco. Moreover, the use of a protease combined with a depolymerase may not be an economical approach to protein removal.
It is desirable to provide a technique for protein removal from tobacco material which does not cause a physical degradation of the tobacco structure and is economical and efficient. Tobacco material includes tobacco solids and any solid form of tobacco including cured tobacco.
It is also desirable to provide an efficient and cost effective process for removal of solubilized polypeptides (which include proteins) from an aqueous extract of tobacco, before the extract is added back to tobacco material. In the aforementioned patent of Gaisch et al., this was accomplished by assimilation of protein fragments by yeast. Clapp et al. (U.S. Pat. No. 4,941,484) describes the use of ultrafiltration to remove high molecular weight compounds (e.g. proteins) from an aqueous extract of tobacco before the extract is added back to protein-reduced tobacco. The process of Gaisch et al. is complicated by the requirement to ferment the aqueous extract in the presence of yeast. The ultrafiltration process of Clapp et al. requires the use of ultrafiltration apparatus and may not be useful for the removal of proteins or polypeptides outside the cut-off values of the ultrafiltration membrane employed in the procedure.
It is also known to treat aqueous extracts of tobacco with solid adsorbents which will remove polyphenols from the extract according to the patent of Jacin, et al. (U.S. Pat. No. 3,561,451). Such adsorbents include alumina and polyamide which are not useful for removal of solubilized protein or polypeptides from the aqueous extract. Heretofore, there were no adsorbents known to be useful for removal of the polypeptides found in a tobacco extract in commercial batch processing.